Ovulation and Fertile Window Calculator
If you are trying to get pregnant, knowing roughly when you ovulate helps you time intercourse. Ovulation usually happens about 14 days before your next period. The days from five days before ovulation up to the day after are your fertile window, when pregnancy is most likely.
Last reviewed:
- Estimated ovulation day
- 15 November 2026
- Fertile window
- 10 November 2026 to 16 November 2026
- Next period expected
- 29 November 2026
- These are estimates based on a regular cycle. Ovulation can vary from cycle to cycle. This tool must not be used as a method of contraception. Speak to a doctor if you have irregular cycles or difficulty conceiving.
Next three cycles
| Period starts | Ovulation | Fertile window |
|---|---|---|
| 29 November 2026 | 15 November 2026 | 10 November 2026 – 16 November 2026 |
| 27 December 2026 | 13 December 2026 | 8 December 2026 – 14 December 2026 |
| 24 January 2027 | 10 January 2027 | 5 January 2027 – 11 January 2027 |
Enter the first day of your last period and the usual length of your cycle to see your estimated dates for the next three cycles. These are estimates, and this tool should not be used to avoid pregnancy.
How to use this calculator
- Enter the first day of your last period.
- Enter your average cycle length in days.
- Read the estimated ovulation day, fertile window and next period.
- Use the table to plan the next three cycles.
Formula and variables
Next period = Last period start + Cycle length Ovulation = Next period − 14 days Fertile window = Ovulation − 5 days to Ovulation + 1 day
- 14 days - the typical length of the phase between ovulation and the next period.
- Fertile window - sperm can survive up to about five days, and the egg lives about a day.
Worked example
A woman's last period began on 1 November 2026 and her cycle is 28 days long.
| Last period and cycle | 1 November 2026, 28 days |
|---|---|
| Ovulation | 15 November 2026 |
| Fertile window | 10 to 16 November 2026 |
| Next period | 29 November 2026 |
- Next period = 1 November + 28 days = 29 November.
- Ovulation = 29 November − 14 days = 15 November.
- The fertile window runs from 10 to 16 November.
What changes the result
- Cycle regularity. The estimate is better for regular cycles. Stress, illness and travel can move ovulation.
- Cycle length. Longer cycles ovulate later, since the phase after ovulation stays about the same.
- Age and health. Conditions such as PCOS or thyroid problems can make cycles irregular.
Benefits and practical uses
- Plan around your fertile days.
- Track your cycle across months.
- Know when your next period is due.
Assumptions and limitations
- Regular cycles with ovulation 14 days before the next period.
- Dates are estimates and can vary each month.
- This tool is not a contraceptive method.
Frequently asked questions
How do I calculate my ovulation date?
Count your cycle length from the first day of your last period, then subtract 14 days from the date of your next period.
How long is the fertile window?
About six days, from five days before ovulation to one day after.
Can I use this to avoid pregnancy?
No. Ovulation can shift, and this estimate is not reliable enough for contraception. Speak to a doctor about contraception.
What if my cycle is irregular?
The estimate may be wrong. A doctor can advise on tracking methods such as ovulation test kits or ultrasound monitoring.
When should I see a doctor?
If you have been trying for 12 months (or 6 months if over 35) without success, or your cycles are very irregular, talk to a doctor.
References
Page last reviewed on . This calculator gives estimates for planning only; it is not financial advice and not an offer from any lender.